1. Field of the Invention
The present invention relates to a light pulse measuring apparatus capable of measuring optically with a high precision an extremely short light pulse width of picosecond order of the light which is emitted from a light source means such as a laser, and a light pulse width measuring method using the apparatus.
2. Related Background Art
Traditionally, there have been proposed various optical apparatuses utilizing nonlinear optical crystal for attempting the measurement of an extremely short light pulse width of approximately picosecond of the light beam which is emitted from a light source.
FIG. 1 is a schematic view showing the principal part of a conventional light pulse width measuring apparatus using an SHG crystal (KDP) as a nonlinear optical crystal capable of producing a second harmonic generation (SHG).
In FIG. 1, a light beam having a frequency .omega. as light to be measured, which is emitted from a light source means 101 is divided by a beam splitter 102 into two light beams, reflection light LR and transmitting light LT. Of these light beams, the transmitting light LT is reflected by a fixed mirror 104 to return to the original light path. Also, the reflection light LR is reflected by a movable mirror 103 in the direction of the optical axis with a varied length of optical path as compared with the transmitting light LT (with a relative time difference .tau. provided) to return to the original light path. Then, the reflected light and transmitting light are mixed by the beam splitter 102. At this juncture, the mirror 103 is shifted for a predetermined amount to give a known relative time difference to the transmitting light and the reflection light.
In this way, the two light beams enter the nonlinear optical crystal (SHG crystal) 105 which can produce a second harmonic generation (SHG) such as KDP.
A relative positional relation between the polarization orientation of the two light beams and the crystal axis of the SHG crystal is arranged to satisfy the phase matching condition required to product the SHG, thereby to enable the highly efficient second harmonic generation (SHG) of frequency 2.omega.. Then, the light beam of frequency .omega. is cut by an .omega. cut filter 106 to allow only the light beam of the 2.omega. frequency to be transmitted for the detection by a photodetector 107. The generation efficiency of the SHG of the 2.omega. frequency (second harmonic generation) from the SHG crystal 105 becomes dependent on a correlation function concerning the time of the two light beams LT and LR.
Then, one of the mirrors 103 is sequentially shifted in the direction of the optical axis to vary the length of its optical path from zero to a value corresponding to one pulse or more, and the luminous intensity of the SHG is then measured by the photodetector 107 to obtain the autocorrelation function of light pulse of the light to be measured. Thus, by using arithmetic means, the pulse shape of the light to be measured is obtained thereby to secure the light pulse width of the light to be measured.
In the light pulse width measuring apparatus shown in FIG. 1 utilizing an SHG crystal, it is necessary to establish each element so that the light beam can satisfy the phase matching condition when the light beam emitted from a light source means enters the SHG crystal. Consequently, it is extremely difficult to assemble and adjust each of the elements with desired accuracy.
There is also a problem that the SHG crystal must be cut out with a high precision in a desired crystal orientation in order to obtain an SHG crystal having a desirable accuracy. Hence an extremely difficult manufacturing is required.